reserve T, X, Y for Subset of MC-wff;
reserve p, q, r, s for Element of MC-wff;
reserve T, X, Y for Subset of MC-wff;
reserve p, q, r for Element of MC-wff;
reserve T, X, Y for Subset of MC-wff;
reserve p, q, r for Element of MC-wff;

theorem Th84:
  (Nes (p => q)) => ((Nes p) => (Nes q)) in CnS4 (X)
proof
  T is S4_theory & X c= T implies (Nes (p => q)) => ((Nes p) => (Nes q))
  in T;
  hence thesis by Def23;
end;
